This invention relates to a polymeric additive which, when added to an oil will increase its viscosity, particularly at higher temperatures, and to an oil composition comprising said polymeric additive. More particularly, this invention relates to a polymeric additive of the star polymer variety and to lubricating oil compositions comprising the same.
As is well known, the viscosity of lubricating oils varies with temperature, and, since lubricating oils generally incur a relatively broad temperature range during use, it is important that the oil not be too viscous (thick) at low temperatures nor too fluid (thin) at higher temperatures. As is well known, variation in the viscosity-temperature relationship of an oil is indicated by the so-called viscosity index (VI). The higher the viscosity index, the less the change in viscosity with temperature. In general, the viscosity index is a function of the oils viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C.
Heretofore several methods have been proposed for improving the rheological properties of lubricating oil compositions. Generally these methods involve the use of a polymeric additive. Early work involved the use of polymeric additives such as those taught in U.S. Pat. Nos. 3,554,911; 3,668,125; 3,772,196; 3,775,329 and 3,835,053. The polymeric additives taught in this series of U.S. patents are, generally, hydrogenated, substantially linear polymers of conjugated dienes which may, optionally, also contain monomeric units of a monoalkenyl aromatic hydrocarbon. Polymers of the type disclosed in this series of U.S. patents are typically prepared via the anionic solution polymerization of the monomers followed by hydrogenation. The process involves polymerizing a conjugated diene and optionally, a monoalkenyl aromatic hydrocarbon, in solution, in the presence of an anionic initiator to form an unsaturated, so-called living polymer. Examples of hydrogenated substantially linear polymers which are commercially used as VI improvers include hydrogenated styrene/butadiene and hydrogenated styrene/isoprene copolymers.
More recently, it has been discovered that certain so-called star-shaped polymers such as those disclosed in U.S. Pat. Nos. 4,077,893; 4,116,917; 4,141,847; 4,156,673 and 4,427,834, are effective VI improvers for lubricating oil compositions. In general, all or at least most of the arms contained in the so-called star-shaped polymers will be either homopolymers or copolymers of conjugated dienes or copolymers of one or more conjugated dienes and one or more monoalkenyl aromatic hydrocarbons. In certain such additives, however, one or more arms will either be modified or different. Polymeric additives wherein all of the arms are either a hydrogenated homopolymer or copolymer of one or more conjugated dienes or selectively hydrogenated copolymers of conjugated dienes and monoalkenyl arenes are disclosed in U.S. Pat. Nos. 4,116,917 and 4,156,673. Star-shaped polymers of the type disclosed in U.S. Pat. Nos. 4,116,917 and 4,156,673 which are modified to incorporate functional groups imparting dispersant characteristics are taught in U.S. Pat. Nos. 4,077,893 and 4,141,847. In the process disclosed in U.S. Pat. No. 4,077,893 hydrogenated star-shaped polymers are reacted first with an unsaturated carboxylic acid or derivative thereof and then with an alkane polyol. In the process disclosed in U.S. Pat. No. 4,141,847 hydrogenated star-shaped polymers are reacted first with an unsaturated carboxylic acid or derivative thereof and then with an amine. In the star-shaped polymeric additives taught in U.S. Pat. 4,427,834 star-shaped polymers such as those taught in U.S. Pat. Nos. 4,116,917 and 4,156,673 are, effectively, modified by incorporating an arm prepared by polymerizing a nitrogen containing polar compound monomer.
As is well known in the prior art, thickening efficieny of the polymeric additive is an important, and frequently the principal, consideration in its selection for use as a VI improver. Particularly, polymeric additives which significantly increase the high temperature kinematic viscosity without significantly increasing the low temperature kinematic viscosity are sought. In general, the thickening efficiency of any given polymeric additive will vary with polymer composition and structure but will, generally, increase with increased molecular weight. The ability of the polymeric additive to maintain an increase in viscosity after the same has been subjected to mechanical shear is also an important consideration in the selection of a polymeric additive for use as a VI improver. In general, lower molecular weight polymeric additives exhibit better mechanical shear stability than do the high molecular weight polymeric additives. Improved mechanical shear stability is, then, generally achieved at the expense of thickening efficiency. As a result, increased concentration of lower molecular weight polymeric additives will, generally, be required to achieve any given viscosity increase when lower molecular weight polymeric additives are used as a VI improver.
Another property which is frequently considered in the selection of a particular polymeric additive for use as a viscosity index improver is the high temperature, high shear rate (HTHSR) viscosity of the oil blend comprising the polymeric viscosity index improver. Heretofore, higher HTHSR viscosities were, generally, sought, although, as a practical matter, the HTHSR value associated with the desired balance of thickening efficiency and mechanical shear stability was generally accepted. In general, these HTHSR values have been relatively high. More recently, however, and as indicated in a paper entitled "The Effects of Engine Oil Viscosity and Composition on Fuel Efficiency", Clevinger, Carlson and Kleiser, which was presented at an SAE Fuels and Lubricants Meeting and Exposition in October, 1984, it has been learned that VI improvers having lower HTHSR viscosities lead to improved fuel efficiencies. In light of this recent discovery, the need for a viscosity index improver having a good balance between thickening efficiency and mechanical shear stability and also having a relatively low HTHSR viscosity thereby permitting the production of a multigrade oil having improved fuel efficiency is believed readily apparent.